Process for the preparation of a catalyst capable of promoting the oxidative conversion of methane into higher hydrocarbons and use of catalyst

ABSTRACT

A process is provided for the preparation of a catalyst which is particualrly active and selective in the oxidative conversion of methane into higher hydrocarbons and especially into C 2   +  hydrocarbons. 
     In the process, an aqueous phase containing in solution cations of at least one metal of the lanthanide group and cations of at least one alkaline-earth metal, is brought into contact with a sufficient quantity of a source of carbonate ions or hydroxide ions to form a coprecipitate of carbonates and/or of hydroxycarbonates of the metals, the pH of the resulting reaction medium is brought to a value higher than 8, a coprecipitate is separated from the reaction medium, subjected to a washing operation, then to a drying operation and the washed and dried coprecipitate is subjected to calcination.

BACKGROUND OF THE INVENTION

1.) Field of the Invention

The invention relates to a process for the preparation of a catalystcapable of promoting the oxidative conversion of methane into C₂ ⁺hydrocarbons with a high activity and selectivity.

2.) Background of the Related Art

There is at present no industrial-scale process which makes it possibledirectly to convert methane, a hydrocarbon which has a limited field ofapplication, into higher hydrocarbons such as ethylene, which arecapable of many utilizations in various fields of the chemical industry.

However, there are many catalyst systems which have been proposed withinrecent years, for converting methane, in the presence of oxygen, intohigh hydrocarbons and especially into C₂ ⁺ hydrocarbons, which aremixtures of ethylene, ethane and of small quantities of hydrocarbonscontaining at least three carbon atoms in the molecule, this operationof conversion of methane into higher hydrocarbons being commonly calledan oxidative conversion or oxidative coupling of methane.

Thus, for the selective conversion of methane into C₂ ⁺ in the presenceof oxygen, reference DE-A-3,237,079 proposes to employ a catalyst basedon PbO and on SiO₂, and reference EP-A-0,196,541 describes the use of anLi/MgO catalyst, in the formulation of which the lithium is generallyintroduced in the form of carbonate. Such catalysts rapidly lose theiractivity at the temperatures needed to activate methane, which are ofthe order of 750° C., given that at these temperatures PbO is eliminatedby sublimation and lithium carbonate is unstable and decomposes.

In reference EP-A-0,189,079, a catalyst based on rare earths in the formof oxides is proposed for performing the conversion of methane in thepresence of oxygen. The results of the tests presented in this referenceshow that the use of a catalyst of this kind does not make it possibleto reach a sufficient yield of ethylene and ethane. In fact, theselectivity for these two hydrocarbons is high at a low degree ofconversion of methane, but it quickly decreases when the conversionincreases. The use of the said catalyst appears therefore to be oflittle economic interest for an industrial application of the oxidativeconversion of methane into higher hydrocarbons.

Reference WO-A-86/07,351 proposes to perform the conversion of methanein the presence of oxygen by employing a catalyst based on rare-earthoxides doped by adding oxides of metals of groups IA and IIA of thePeriodic Table of the Elements, and this makes it possible to work withspace velocities of the methane/oxygen mixture which are markedly higherthan those envisaged in reference EP-A-0,189,079 and to attain higherdegrees of conversion of methane with improved selectivities forhydrocarbons.

It has been found that a mixture of oxides of metals of groups IA andIIA and of rare-earth metals was not the most appropriate form forconstituting an active and selective catalyst for oxidative conversionof methane into C₂ ⁺ hydrocarbons, and that it was possible to obtain acatalyst based on the said metals exhibiting improved performance, whenthe catalyst preparation process passed through a stage ofcoprecipitation of these metals in the form of carbonates and/orhydroxycarbonates.

SUMMARY OF THE INVENTION

The invention proposes, therefore, a particular process for thepreparation of a catalyst containing at least one metal of thelanthanide group and at least one alkaline-earth metal, which makes itpossible to obtain a catalyst which is particularly active and selectivein the oxidative conversion of methane into higher hydrocarbons andespecially into C₂ ⁺ hydrocarbons.

The process according to the invention is characterized in that anaqueous phase containing in solution cations of at least one metal ofthe lanthanide group and of at least one alkaline-earth metal is broughtinto contact with a sufficient quantity of a source of carbonate ionsand optionally of a source of hydroxide ions to form a coprecipitate ofcarbonates and/or of hydroxycarbonates of the said metals, containing atleast 5% by weight of carbonates and to bring the pH of the resultingreaction medium to a value higher than 8, the coprecipitate is separatedfrom the reaction medium and the said coprecipitate is subjected to awashing operation and then to a drying operation and the washed anddried coprecipitate is subjected to a calcination at a temperature ofbetween 400° C. and 1000° C.

Advantageously, before separating the coprecipitate from the reactionmedium, the said reaction medium containing the coprecipitate is kept ata temperature ranging from 60° C. to 150° C. for a period of between 30minutes and 30 hours and preferably between 2 hours and 20 hours.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The metals of the lanthanide group which are capable, according to theinvention, of providing some of the cations in solution in the aqueousphase are the metals of the Periodic Table of the Elements which havethe atomic numbers 57 and 59 to 71, the said metals being especiallysuch as lanthanum, praseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, erbium, holmium, thulium, ytterbium andlutetium.

The alkaline-earth metals capable, according to the invention, ofproviding the cations used in combination with the cations of themetal(s) of the lanthanide group are the metals of group IIA of thePeriodic Table of the Elements including beryllium, magnesium, calcium,strontium and barium.

The respective proportions of cations of the metal(s) of the lanthanidegroup and of cations of the alkali-earth metal(s) in the aqueous phasemay vary quite widely. In particular, the respective weight percentagesL of the metal(s) of the lanthanide group and E of the alkaline-earthmetal(s) in the total of these two types of metals are such that 6≦L≦98and 2≦E≦94 with L+E=100%.

Cations of one or more metals other than the metals of the lanthanidegroup and alkaline-earth metals, especially scandium, yttrium andlithium may also be present in the aqueous phase in addition to thecations of the alkaline-earth metals and the cations of the lanthanidegroup. The quantity of these additional cations represents up to 30% andpreferably up to 15% of the total weight of the cations originating fromthe metal(s) of the lanthanide group and from the alkaline-earthmetal(s).

The source of the cations which are present in the aqueous phaseconsists of water-soluble compounds, for example chlorides, of theabovementioned metals.

The source of carbonate ions may be chosen from the various carbonateswhich are water-soluble in the concentrations employed, especiallyalkali metal carbonates such as sodium carbonate and potassiumcarbonate, ammonium carbonate and quaternary ammonium carbonates.

Similarly, the source of hydroxide ions may be chosen from the varioushydroxides which are water-soluble in the concentrations employed,especially alkali metal hydroxides such as sodium and potassiumhydroxides, and quaternary ammonium hydroxides.

The proportion of source of carbonate ions which is employed by itselfor the respective proportions of the sources of carbonate ions and ofhydroxide ions are chosen in particular so that the coprecipitateresulting from bringing the said carbonate or carbonate and hydroxideions into contact with the cations present in the aqueous phase maycontain 5% to 100% by weight of carbonates and so that the pH of thereaction medium may be brought to a value ranging from 9.5 to 13.5.

The cations present in the aqueous phase may be brought into contactwith the carbonate ions or with the carbonate ions and the hydroxideions giving rise to the coprecipitate of carbonates or of carbonates andhydroxycarbonates in any appropriate manner. For example, the source ofcarbonate ions, when employed by itself, may be added, in aqueoussolution, to the aqueous phase containing the cations, it being possiblefor the said addition to be performed in one or more fractions or elsecontinuously. When a source of carbonate ions and a source of hydroxideions are employed together to form the coprecipitate, the source ofhydroxide ions and the source of carbonate ions may be added, in aqueoussolution, either successively or mixed or else simultaneously andseparately, to the aqueous phase containing the cations, it beingpossible for each addition to be performed in one or more fractions orelse continuously.

An advantageous way of bringing the cations present in the aqueous phaseinto contact with the carbonate ions or with the carbonate ions and thehydroxide ions giving rise to the coprecipitate consists in continuouslymixing the aqueous phase containing the cations with an alkaline aqueousphase containing, in solution, either the source of carbonate ions orthe source of carbonate ions and the source of hydroxide ions, in theappropriate concentrations, while controlling the flow rates of theaqueous phases brought together so that the coprecipitate formed maycontain at least 5% by weight of carbonates and that the pH of thereaction medium formed may be maintained at a substantially constantvalue throughout the coprecipitation, the said value being higher than 8and preferably ranging from 9.5 to 13.5.

The coprecipitate resulting from bringing the cations of the metals suchas those referred to above into contact with the carbonate ions or withthe carbonate ions and the hydroxide ions and optionally subjected tothe stage of being kept at a temperature of 60° C. to 150° C. isseparated from the reaction medium by any known method, for examplefiltration or centrifuging, and is then subjected to a washing withdistilled or demineralized water until the interfering ions arecompletely eliminated, for example chloride ions, introduced by thesources of cations and is dried in a conventional manner at temperaturesranging, for example, from 60° C. to 90° C.

The calcination of the dried coprecipitate is carried out by heating thesaid coprecipitate to temperatures of between 400° C. and 1000° C. asindicated above and preferably ranging from 450° C. to 800° C., it beingpossible for the said heating to be carried out in air or in inertatmosphere.

The calcined coprecipitate is shaped by any known technique, for exampleby a pelleting technique, to form the catalyst which can be employed incatalytic reactors.

The product whose preparation has just been described is a catalystwhich is particularly active and selective for the conversion ofmethane, in the presence of oxygen, into high hydrocarbons andespecially into C₂ ⁺ hydrocarbons.

The methane which is subjected to the oxidative catalytic conversion maybe pure methane or else methane containing up to 10% by volume ofethane, as is the case with industrial natural gas.

The oxygen reacted with the methane is preferably pure oxygen, becausethe use of air as a source of oxygen, although possible, demands aseparation of nitrogen before the unconverted methane is recycled.

The methane and the oxygen may be brought into contact with the catalysteither in the form of separate streams or in the form of a preformedmixture. The quantities of methane and of oxygen which are broughttogether are such that the molar ratio of methane to oxygen may have avalue ranging from 1 to 20 and preferably from 2 to 10.

The catalyst prepared according to the invention makes it possible touse the methane conversion reaction at temperatures of between 600° C.and 1100° C. without catalyst decomposition. According to the invention,preferred temperatures for this reaction range from 700° C. to 900° C.

The pressures which may be employed for using the said reaction are notcritical. They may be especially between approximately 1 andapproximately 50 bars and may preferably lie between 1 and 20 bars.

The space velocity of the gaseous mixture of methane and oxygen incontact with the catalyst, expressed in liters of gaseous mixture pergram of catalyst per hour may, according to the invention, have a valueof between 3 and 1000. Preferred space velocities have values rangingfrom 5 to 300.

The invention is illustrated by the following examples, given withoutany limitation being implied.

EXAMPLE 1

An aqueous solution of barium cations and of lanthanum cations wasprepared by dissolving 0.25 moles of BaCl₂.2H₂ O and 0.125 moles ofLaCl₃.6H₂ O in 1 liter of distilled water and the solution obtained wasplaced in a 2-1 round bottom flask purged with a stream of nitrogen.

An aqueous solution containing 0.25M/l of NaOH and 0.25M/l of Na₂ CO₃was added dropwise to the contents of the round bottom flask, kept atambient temperature and with continuous stirring, so as to precipitatethe barium and lanthanum cations in the form of a coprecipitate ofcarbonate and of hydroxycarbonate, the rate of addition of the saidsolution being controlled so as to obtain a reaction medium with a pHequal to 10 after an addition period of 2 hours.

The resulting reaction medium containing the coprecipitate formed wasthen heated to 90° C. and kept at this temperature, the stirring beingcontinued, for a period of 16 hours.

The coprecipitate was then separated from the reaction medium byfiltration, was then washed with distilled water until the chlorideswere completely eliminated and was finally dried at 60° C.

The dried precipitate was then subjected to a calcination at 500° C. inthe presence of air.

The composition of the calcined product, defined as metal oxides and ascarbonate equivalents expressed as CO₂, is given below as a percentageby weight:

La₂ O₃ : 63.6%, BaO: 7.2% and CO₂ : 29.2%.

EXAMPLE 2

The operation was carried out as shown in Example 1, starting with anaqueous solution of barium cations and of lanthanum cations, which wasprepared by dissolving 0.278 moles of BaCl₂.2H₂ O and 0.139 moles ofLaCl₃.6H₂ O in 1 liter of distilled water.

The composition of the product obtained by calcination of the driedcoprecipitate, defined as metal oxides and as carbonate equivalentsexpressed as CO₂, was the following, in percentage by weight: La₂ O₃ :55%, BaO: 27% and CO₂ : 18%.

EXAMPLE 3

An aqueous solution of barium cations and of lanthanum cations wasprepared by dissolving 0.6 moles of BaCl₂.2H₂ O and 0.2 moles ofLaCl₃.6H₂ O in 1 liter of distilled water.

An aqueous precipitating solution was also prepared by dissolving 3.2moles of NaOH and 8×10⁻³ moles of Na₂ CO₃ in 1 liter of distilled water.

The aqueous solution of cations, on the one hand, and the precipitatingaqueous solution for precipitating the cations in the form of acoprecipitate of carbonate and of hydroxycarbonate, on the other hand,were introduced continuously, simultaneously and separately into a2-liter round bottom flask purged with a stream of nitrogen, theoperation being carried out at ambient temperature and with stirring,the flow rates of the solutions introduced into the round bottom flaskbeing controlled to maintain the pH of the reaction medium resultingfrom bringing the said solutions into contact at a constant value equalto 13.

After the introduction of 800 ml of aqueous solution of cations into theround bottom flask, the addition of the said solution and that of theprecipitating solution were stopped and the reaction medium was stillkept stirred until the precipitation of the cations was complete.

The reaction medium containing the coprecipitate was then heated to 80°C. and kept at this temperature for a period of 5 hours, still beingstirred.

The coprecipitate was then separated from the reaction medium byfiltration, it was then washed with distilled water until the chlorideswere completely eliminated and was finally dried at 60° C.

The dried precipitate was then subjected to a calcination at 500° C. inthe presence of air. The calcined product formed consisted of a bariumlanthanum oxycarbonate.

The composition of the said calcined product, defined as shown inExample 1, was the following, in percentage by weight: La₂ O₃ : 38.8%,BaO: 53.9% and CO₂ : 7.3%.

EXAMPLE 4

An aqueous solution of magnesium cations and of samarium cations wasprepared by dissolving 0.75 moles of MgCl₂.6H₂ O and 0.25 moles ofSmCl₃.6H₂ O in 1 liter of distilled water.

An aqueous precipitating solution was also prepared by dissolving 1.6moles of NaOH and 0.01 moles of Na₂ CO₃ in 1 liter of distilled water.

The aqueous solution of cations, on the one hand, and the precipitatingaqueous solution for precipitating the cations in the form of acoprecipitate of carbonate and of hydroxycarbonate, on the other hand,were introduced dropwise, simultaneously and separately, into a 2-literround bottom flask purged with a stream of nitrogen, the operation beingcarried out at constant temperature and with stirring, the flow rates ofthe solutions introduced into the round bottom flask being controlled soas to maintain the pH of the reaction medium resulting from bringing thesaid solutions into contact at a constant value equal to 10.

After the introduction of 800 ml of aqueous solution of cations into theround bottom flask, the addition of the said solution and that of theprecipitating solution were stopped and the reaction medium was stillkept stirred until the precipitation of the cations was complete.

The reaction medium containing the coprecipitate was then heated to 70°C. and kept at this temperature, stirred continuously, for a period of18 hours.

The coprecipitate was then separated from the reaction medium byfiltration, was then washed with distilled water until the chlorideswere completely eliminated and was finally dried at 60° C.

The dried precipitate was then calcinated at 500° C. in the presence ofair.

The resulting calcined product consisted of a magnesium samariumoxycarbonate.

The composition of the said calcined product, defined as shown inExample 1, was the following, in percentage by weight: Sm₂ O₃ : 48.7%,MgO: 34.4% and CO₂ : 16.9%.

EXAMPLE 5

An aqueous solution of barium cations and of samarium cations wasprepared by dissolving 0.75 moles of BaCl₂.2H₂ O and 0.25 moles ofSmCl₃.6H₂ O in 1 liter of distilled water.

An aqueous precipitating solution was also prepared by dissolving 10moles of NaOH and 0.12 moles of Na₂ CO₃ in 1 liter of distilled water.

The aqueous solution of cations, on the one hand, and the precipitatingaqueous solution for precipitating the cations in the form of acoprecipitate of carbonate and of hydroxycarbonate, on the other hand,were introduced dropwise, simultaneously and separately, into a 2-literround bottom flask purged with a stream of nitrogen, the operation beingcarried out at ambient temperature and with stirring, the flow rates ofthe solutions introduced into the round bottom flask being controlled soas to maintain the pH of the reaction medium resulting from bringing thesaid solutions into contact at a constant value equal to 13.

After the introduction of 800 ml of aqueous solution of cations into theround bottom flask, the addition of the said solution and that of theprecipitating solution were stopped and the reaction medium was stillkept stirred until the precipitation of the cations was complete.

The reaction medium containing the coprecipitate was then heated to 70°C. and kept at this temperature, stirred continuously, for a period of18 hours. The coprecipitate was then separated from the reaction mediumby filtration, was then washed with distilled water until the chlorideswere completely eliminated and was finally dried at 60° C.

The dried precipitate was then calcined at 500° C. in the presence ofair. The resulting calcined product consisted of a barium samariumoxycarbonate.

The composition of the said calcined product, defined as shown inExample 1, was the following, in percentage by weight: Sm₂ O₃ : 53.3%,BaO: 25% and CO₂ : 21.7%.

EXAMPLE 6

The calcined solids obtained in Examples 1 to 5 according to theinvention were employed as catalysts in trials of oxidative conversionof methane into higher hydrocarbons.

Control tests were also performed by employing the following products ascatalysts:

Product A: Lanthanum oxide La₂ O₃ containing 7.2% by weight of BaO andobtained by impregnating lanthanum oxide with the appropriate quantityof aqueous solution of barium nitrate, followed by drying of theimpregnated product at 110° C. for 12 hours and finally calcination at600° C. for 4 hours to provide lanthanum oxide (La₂ O₃) particles havinga surface layer containing barium oxide (BaO).

Product B: Barium carbonate obtained by precipitation, by mixing equalvolumes of a 0.2M aqueous solution of BaCl₂.2H₂ O and of a 0.2M aqueoussolution of Na₂ CO₃, followed successively by isolating the precipitateby filtration, washing the precipitate with distilled water until thechlorides were eliminated, drying at 60° C. for 24 hours and finallycalcination at 600° C. for 4 hours in air to provide barium carbonate(BaCO₃).

Product C: Lanthanum oxycarbonate obtained by precipitation, by mixingequal volumes of a 0.2M aqueous solution of LaCl₃.6H₂ O and of a 0.2Maqueous solution of sodium carbonate, followed in succession byisolating the precipitate by filtration, washing the precipitate withdistilled water until, the chlorides were eliminated, drying at 60° C.for 24 hours and finally calcination at 600° C. for 4 hours in air toprovide lanthanum oxycarbonate, La₂ O(CO₃)₂.

The trials of oxidative catalytic conversion of methane were carried outas follows.

The operation was carried out with a device comprising a tubular reactormounted in a furnace provided with a temperature control system, one ofthe ends of the reactor being connected to a delivery line for gaseousmixture, equipped with a flowmeter, and the other end of the reactorbeing connected to a condenser, maintained at 0° C. and itself connectedto a sampling system of a chromatographic analyser.

A mixture of 2 g of the chosen catalyst, the said catalyst being in theform of particles which have a particle size range from 1 to 2 mm, andof quartz grains in a volumetric ratio of the catalyst to the quartzequal to 1:2, was placed in the tubular reactor and held in place in thesaid reactor by quartz lame plugs. In the reactor, the catalyst mixtureoccupied a position which placed the said mixture substantially in themiddle of the heating zone of the furnace when the reactor was placed inthe furnace.

The furnace containing the reactor charged with the catalyst mixture wasbrought up to the temperature required for the trial, and a gaseousmixture, preheated to 450° C. and consisting of methane and of pureoxygen in an appropriate molar ratio was injected into the reactorthrough the gas delivery line, the said gaseous mixture travelling at aspace velocity controlled by the flowmeter fitted on the gas deliveryline. At the exit of the reactor the gaseous reaction mixture resultingfrom the conversion was cooled in the condenser and was then directedtowards the sampling system of the chromatographic analyser for thepurpose of qualitative and quantitative analyses.

The operating conditions specific to each trial and the results obtainedare collated in the table below.

                                      TABLE                                       __________________________________________________________________________    SOURCE    MOLAR                                                                              CONVERSION                                                                              SPACE  PERCENTAGE  SELECTIVITY                       OF THE    RATIO                                                                              TEMPERATURE                                                                             VELOCITY                                                                             CONVERSION  (% CARBON)                        CATALYST  CH.sub.4 :O.sub.2                                                                  (°C.)                                                                            (1 g.sup.-1 h.sup.-1)                                                                CH.sub.4                                                                         O.sub.2                                                                          C.sub.2 H.sub.4                                                                  C.sub.2 H.sub.6                                                                  C.sub.3                                                                          C.sub.2.sup.+                                                                    CO CO.sub.2                 __________________________________________________________________________    EXAMPLE 1 6    752       30.1   21.5                                                                             99.8                                                                             37.5                                                                             31.1                                                                             3.4                                                                              72 3  25                       EXAMPLE 2 6    738       33.1   18.9                                                                             99.9                                                                             42.5                                                                             25.7                                                                             2.3                                                                              70.5                                                                             3.4                                                                              26.1                     EXAMPLE 3 6    750       30.3   16 91.7                                                                             17.2                                                                             22.2                                                                             2.5                                                                              41.9                                                                             2.6                                                                              55.3                     EXAMPLE 3 6    837       180    16.3                                                                             92.4                                                                             23.6                                                                             36.1                                                                             3.2                                                                              62.9                                                                             4.5                                                                              32.6                     EXAMPLE 4 6    746        5.1   15.7                                                                             99.9                                                                             20.1                                                                             29.3  49.4                                                                             12.3                                                                             38.3                     EXAMPLE 5 6    750       26     18.1                                                                             99.9                                                                             35.2                                                                             30.5                                                                             2.5                                                                              68.2                                                                             3.8                                                                              28                       Product A 6    782       15     19.1                                                                             99.8                                                                             25.6                                                                             30.9                                                                             3.5                                                                              60 6.9                                                                              33.1                     Product B 10   800         5.1  10.1                                                                             84.6                                                                             31.7                                                                             28.6  60.3                                                                             4  35.7                     Product C 10   750        5.1   10.4                                                                             99.4                                                                             22.4                                                                             22.1                                                                             2.1                                                                              46.6                                                                             5.8                                                                              47.6                     __________________________________________________________________________

Comparison of the results which appear in the Table brings intoprominence the improved performance of the catalysts according to theinvention. In particular, for comparable contents of barium oxide, thecatalyst prepared in Example 1 results in a degree of methane conversionwhich is slightly higher with a selectivity for C₂ ⁺ hydrocarbons whichis substantially improved when compared with what is obtained whenemploying Product A as catalyst, that is to say a product containing theelements barium and lanthanum in the form of oxides. Furthermore, thecatalysts according to the invention result in degrees of methaneconversion which are substantially higher than those obtained incomparable conditions with catalysts consisting of barium carbonate(Product B) or of lanthanum carbonate (Product C), without theselectivity for hydrocarbons being affected. Moreover, the catalystsaccording to the invention make it possible to work at high spacevelocities.

We claim:
 1. A process for the preparation of a catalyst which containsat least one metal of the lanthanide group and at least onealkaline-earth metal and wherein at least part of said metals are in theform of carbonates and/or oxycarbonates, said process comprising,1)contacting:(a) an aqueous phase containing in solution cations of atleast one metal of the lanthanide group having the atomic numbers 57 and59 to 71 in the Periodic Table of the Elements and cations of at leastone alkaline-earth metal, with (b) a sufficient quantity of a source ofcarbonate ions and optionally of a source of hydroxide ions to form acoprecipitate of carbonates and/or of hydroxycarbonates of the saidmetals, containing at least 5% by weight of carbonates and to bring thepH of the resulting reaction medium to a value higher than 8; 2)separating the coprecipitate from the reaction medium and subjectingsaid coprecipitate to a washing operation; 3) drying said coprecipitate;and 4) subjecting said coprecipitate to calcination at a temperature ofbetween 400° C. and 1000° C.
 2. Process according to claim 1, where,before separating the coprecipitate from the reaction medium, thereaction medium containing the coprecipitate is kept at a temperatureranging from 60° C. to 150° C. for a period of between 30 minutes and 30hours.
 3. Process according to claim 1 wherein the metal of thelanthanide group is selected from the group consisting of lanthanum,praseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, erbium, holmium, thulium, ytterbium and lutetium.
 4. Processaccording to claim 1 wherein the alkaline-earth metal or metals areselected from the group consisting of beryllium, magnesium, calcium,strontium and barium.
 5. Process according to claim 1 wherein therespective weight percentages L of the metal(s) of the lanthanide groupand E of the alkaline-earth metal(s) providing the cations present inthe aqueous phase in the total of the said metals are chosen so that6≦L≦98 and 2≦E≦94 with L+E=100%.
 6. Process according to claim 1 whereinin addition to the cations of the metal(s) of the lanthanide group andof the cations of the alkaline-earth metal(s), the aqueous phase alsocontains in solution up to 30% of the total weight of the cationsoriginating from the metal(s) of the lanthanide group and from thealkaline-earth metal(s), of additional cations of at least one othermetal selected from the group consisting of lithium, scandium andyttrium.
 7. Process according to claim 6 wherein the quantity of thesaid additional cations represents up to 15% of the total weight of thecations originating from the metal(s) of the lanthanide group and fromthe metal(s) of the lanthanide group and from the alkaline-earthmetal(s).
 8. Process according to claim 1 wherein the source ofcarbonate ions is chosen from alkali metal carbonates, ammoniumcarbonate and quaternary ammonium carbonates.
 9. Process according toclaim 1 wherein the source of hydroxide ions is chosen from alkali metalhydroxides and quaternary ammonium hydroxides.
 10. Process according toclaim 6 wherein the proportion of source of carbonate ions or therespective proportions of the sources of carbonate ions and of hydroxideions are chosen so that the coprecipitate resulting from bringing thesaid ions into contact with the cations present in the aqueous phase maycontain 5% to 100% by weight of carbonates and so that the pH of thereaction medium may be brought to a value ranging from 9.5 to 13.5. 11.Process according to claim 1 wherein the cations present in the aqueousphase are brought into contact solely with a source of carbonate ionswhich is added in the form of an aqueous solution to the said aqueousphase, the said addition being performed in one or more fractions orcontinuously.
 12. Process according to claim 1 wherein with cationspresent in the aqueous phase are brought into contact with a source ofhydroxide ions and a source of carbonate ions, the said bringing intocontact being carried out by adding the source of hydroxide ions and thesource of carbonate ions, in the form of aqueous solutions, eithersuccessively or mixed or else simultaneously and separately, to theaqueous phase containing the cations, each addition being performed inone or more fractions or else continuously.
 13. Process according toclaim 1 wherein the bringing into contact of the cations present in theaqueous phase with the carbonate ions or with the carbonate ions and thehydroxide ions is performed by continuously mixing the aqueous phasecontaining the cations with an alkaline aqueous phase containing insolution either the source of carbonate ions or the source of carbonateions and the source of hydroxide ions, in the appropriateconcentrations, the flow rates of the aqueous phases brought togetherbeing controlled so that the coprecipitate formed may contain at least5% by weight of carbonates and that the pH of the reaction mediumproduced may be maintained at a substantially constant value throughoutthe coprecipitation, the said value being higher than
 8. 14. Processaccording to claim 1 wherein the coprecipitate separated from thereaction medium is washed with distilled or demineralized water untilthe interfering ions are completely eliminated.
 15. Process according toclaim 1 wherein the washed coprecipitate is dried at temperaturesranging from 60° C. to 90° C.
 16. Process according to claim 1 whereinthe dried coprecipitate is calcined at temperatures ranging from 450° C.to 800° C.
 17. Process for oxidative conversion of methane into higherhydrocarbons in which a gaseous mixture containing methane and oxygen ispassed in contact with a catalyst, at a temperature of between 600° C.and 1100° C., wherein the said catalyst is a catalyst obtained by theprocess according to claim
 1. 18. Process according to claim 17, whereinthe molar ratio of methane to oxygen in the gaseous mixture brought intocontact with the catalyst has a value ranging from 1 to
 20. 19. Processaccording to claim 17 wherein the temperature for implementing theconversion has a value ranging from 700° C. to 900° C.
 20. Processaccording to claim 17 wherein the space velocity of the gaseous mixturebased on methane and oxygen in contact with the catalyst assumes valueswhich, expressed in liters of gaseous mixture per grams of catalyst perhour, are between 3 and
 1000. 21. Process according to claim 17 whereinthe conversion is performed at pressures of between approximately 1 barand approximately 50 bars.
 22. Process according to claim 1, where,before separating the coprecipitate from the reaction medium, thereaction medium containing the coprecipitate is kept at a temperatureranging from 60° C. to 150° C. for a period of between 2 hours and 20hours.
 23. Process according to claim 1, wherein the bringing intocontact of the cations present in the aqueous phase with the carbonateions or with the carbonate ions and the hydroxide ions is performed bycontinuously mixing the aqueous phase containing the cations with analkaline aqueous phase containing in solution either the source ofcarbonate ions or the source of carbonate ions and the source ofhydroxide ions, in the appropriate concentrations, the flow rates of theaqueous phases brought together being controlled so that thecoprecipitate formed may contain at least 5% by weight of carbonates andthat the pH of the reaction medium produced may be maintained at asubstantially constant value throughout the coprecipitation, the saidvalue ranging from 9.5 to 13.5.
 24. Process according to claim 17,wherein the molar ratio of methane to oxygen in the gaseous mixturebrought into contact with the catalyst has a value ranging from 2 to 10.25. Process according to claim 17, wherein the space velocity of thegaseous mixture based on methane and oxygen in contact with the catalystassumes values which, expressed in liters of gaseous mixture per gram ofcatalyst per hour, are between 5 and
 300. 26. Process according to claim17, wherein the conversion is performed at pressures of betweenapproximately 1 bar and 20 bars.
 27. Process according to claim 1,wherein in addition to the cations of the metal(s) of the lanthanidegroup and of the cations of the alkaline-earth metal(s), the aqueousphase also contains in solution additional cations of at least one othermetal selected from the group consisting of scandium, yttrium andlithium.